The invention relates to a device for protection against the presence of transmission errors in an information transmission system, particularly for railroad systems. An interrogation apparatus emits a periodically variable interrogation signal within a specified frequency band consisting of successive frequency ranges. A reply apparatus dampens in each case m of n sub-ranges of each frequency range of the interrogation signal in accordance with an item of information to be transmitted, so that the interrogation signal which is frequency-selectively reflected from the reply apparatus to the interrogation apparatus has characteristic power penetrations in the respective sub-ranges.
Such a system for point by point information transmission between an active interrogation apparatus and a passive reply apparatus is, for example, known from the publication "Microwave System SICARID TCL" of Siemens AG. This system, depending on the use of the system components for vehicle information or distance information, can be used for the transmission of vehicle data or of distance data to a vehicle. The sensitive interrogation apparatus is in each case carefully protected whereas the reply apparatus is exposed to rough surrounding conditions. For this reason, the reply apparatuses are produced as solid cast metal components in which the resonance circuits of the reply apparatus for damping the interrogation signal are formed as hollow resonators. Common resonance frequency displacements of all hollow resonators resulting from temperature fluctuations of the surroundings can, for example, be compensated for in the interrogation apparatus with the aid of a device in accordance with British Letters Patent No. 1,442,914.
However, under unfavorable conditions in which the resonant frequency of a single hollow resonator in the reply apparatus is altered because of gradually appearing corrosion symptoms, the alteration of one of such m resonant frequencies of a sub-range can, under certain conditions, cause an information adulteration which is not recognizable.